Integrated analog signal processing systems are inherently encumbered by custom specifications which if implemented in VLSI (Very Large Scale Integration) designs require costly custom design and fabrication. In contrast, VLSI digital systems are easily implemented in less than full custom designs (e.g., semi-custom and gate arrays) due to the highly structured and repetitive use of elementary logic building blocks. The elementary digital logic blocks can be systematically arranged in regular array structures which can be customized to the required application by interconnecting the elementary digital logic blocks to create the required function. If a standard analog building block can be found which lends itself to implementing a wide variety of analog implementations, a standard cell design for analog circuits can be implemented allowing customized VLSI designs by interconnecting a number of elementary linear analog cells in order to achieve a highly systematic design and chip layout with high functional density.
Transconductance elements or voltage-to-current transducers (VCTs) are essential elements of a wide range of analog signal processing circuits. In order for them to be effective, transconductance elements must operate in a linear fashion across a wide input voltage range. Single MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) transistors will act as VCTs but the output current exhibits a non-linear square-law relationship to the input voltage. Multiple transistor linear transconductance elements have been achieved through several approaches by various circuit techniques which compensate for the non-linear term of the basic transistor I-V relationship. These designs have typically required many transistors and consume large amounts of chip surface area.
Bipolar transconductance elements have been used as the basic building blocks in such circuits as operational amplifiers, comparators, and other analog signal processing circuits. MOS technology is, however, more attractive for VLSI semi-custom designs due to its simpler fabrication requirements and higher packing density than bipolar technology. CMOS (Complementary Metal-Oxide-Semiconductor) technology offers the additional advantage of complementary devices and lower power operation.
Prior art linear transconductance elements implemented in CMOS technology have achieved linear operation over a limited voltage range by linearizing a CMOS differential stage using current differencing so that the nonlinear term of the CMOS transistor I-V curve is eliminated. This approach shown in FIG. 1 provides a fairly high gain linear VCT but at a cost of increased chip surface area usage due to the use of many transistors. Several constant current sources are required to provide the current biasing of the circuit which results in a linear VCT implemented in approximately 10 transistors at a minimum.
Simple linear VCTs have been constructed based on using the well-known CMOS inverter by critically matching the gain parameters of the p-MOS and n-MOS transistors in the fabrication of the circuit as shown in FIG. 2. The advantage here is the elimination of the requirement of constant current sources. Unfortunately, power-supply rejection is very poor, the transconductance cannot be tuned, and a very limited linearity has been achieved in practice due to the difficulty of matching the gain constants (.mu.C.sub.ox W/L) between the p-MOS and n-MOS transistors. The gain of a p-MOS or n-MOS transistor depends on the physical width-to-length ratio W/L of the gate area, the gate oxide capacitance C.sub.ox per unit area, and the effective mobility .mu. of the charge carriers. Since the effective mobility of electron charge carriers is much greater than the effective mobility of hole charge carriers, the gain constant for an n-MOS transistor is much greater than the gain constant of a p-MOS transistor of the same device dimensions. Therefore the gain of the n-MOS and p-MOS transistors can only be matched by using exact ratios of the width-to-length dimensions of the gates. Since the effective mobility of the charge carriers depends upon doping, bias voltages and temperature, good matching between the p-MOS and n-MOS devices will be difficult to achieve in practice.
To alleviate these difficulties, and to provide for a simple CMOS transconductance element which can be used in regular analog arrays, a linear tunable VCT with as few as four transistors is described. The circuit is constructed by effecting current differencing between two composit transistors, each of which is constructed with a p-channel/n-channel pair of MOSFETs, where the gain of the circuit is tunable by applying fixed reference voltages.